C - Lublin
C - Lublin
C - Lublin
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ORGANIC SOIL CONSTITUENTS<br />
Acidic functional groups of soil organic matter (aliphatic and aromatic carboxyls,<br />
sulfoxyls, phenolic, enolic etc.) have very different acidic strength, depending<br />
not only on the kind of the group, but also on its locality. An increase of the pH<br />
of the soil solution leads to the neutralization of these groups thus they become<br />
negatively charged. Surface groups of stronger acidic character (similarly as<br />
stronger acids) are neutralized at lower pH values. The weaker acidic is the group,<br />
its neutralization requires higher pH value. Therefore the higher is the soil pH, the<br />
larger surface charge occurs on organic matter surfaces. These acidic surface<br />
groups, which are located closely to each other, create common electric field surrounding<br />
these groups and within this field their protons become delocalized (proximity<br />
effect). The delocalized protons behave as strong acids and so the neighboring<br />
groups are strongly acidic and the soil organic matter has some negative charge<br />
even at very low pH values. The negative charge at pH around 8.2 of fulvic acids<br />
reaches a few hundreds centimoles per kg and for fulvic acids up to one thousand.<br />
PLANT ROOTS<br />
On surfaces of plant roots, variable charge of carboxylic groups dominate. The<br />
charge of plant roots was found to be closely correlated to uronic acid, present as<br />
polymerized galacturonic acid in pectic substances (Knight et al. 1961). The magnitude<br />
of root charge (CEC) was used for an explanation how various plant species<br />
survive environments with low level of cations availability (Gray et al. 1953), why<br />
plants uptake different proportion of mono and divalent cations (Huffaker and Wallace<br />
1958), how plants compete in nutrient deficient mixed populations (Woodward<br />
et al. 1983), what are the mechanisms of aluminum toxicity (Keltjens 1995).<br />
This may be connected also with the surface charge density. The higher the SCD,<br />
the higher the surface potential and the higher relative adsorption of multivalent<br />
cations. Significant positive correlation between CEC of cell walls and Al sorption<br />
was found by Schmol and Horst (2001), who postulated that Al binding by pectin<br />
matrix is an important step in the expression of Al toxicity. The roots charge is<br />
higher for Al sensitive plants than for Al resistant plants.<br />
REFERENCES<br />
1. Chi Ma and Eggleton, R.A. Cation exchange capacity of kaolinite. Clays and Clay Minerals<br />
47, 174–180.<br />
2. Gray B, Drake M and Colby W G 1953 Potassium competition in grass-legume associations<br />
as a function of root cation exchange capacity. Soil Sci. Soc. Am. Proc. 17,<br />
235-239.<br />
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